A capacitor comprises two insulated conductors, and the capacitance is characterized by the field distribution in the two-body system. If charge Q is placed on one conductor and −Q on the other conductor, the potential difference between the two conducting bodies is proportional to Q and the proportional constant is 1/C where C is called capacitance.
The charge distribution produces an electric field, which in turn causes a potential difference V. The electric field is also affected by the material between the two conductors. When an external field is applied to a dielectric material, the molecules/atoms of the dielectric are polarized to become dipole moments which will produce a net electric field in the opposite direction to the external field, resulting in decreased potential difference. Consequently, the capacitance increases in the presence of dielectric material. The reduction of the electric field due to the presence of dielectric material is characterized by a dielectric constant εr (or relative permittivity), such that the resultant net electric field becomes 1/εr of the external electric field.